[clang][ABI] New C++20 module mangling scheme

Implement a demangleable strong ownership symbol mangling.

 * The original module symbol mangling scheme turned out to be
undemangleable.

 * The hoped-for C++17 compatibility of weak ownership turns out to be
fragile

* C++20 now has better ways of controlling C++17 compatibility

The issue is captured on the ABI list at:
  https://github.com/itanium-cxx-abi/cxx-abi/issues/134

GCC implements this new mangling.

The old mangling is unceremoniously dropped.  No backwards
compatibility, no deprectated old-mangling flag.  It was always
labelled experimental.  (Old and new manglings cannot be confused.)

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D122256